Within current fuel injection systems a pump is used to deliver fuel to the engine cylinders via a high pressure supply line and, in the course of an ‘injection’, fuel enters a cylinder through one or more apertures which otherwise remain closed by seated injection needles. Typically, a vehicle travelling at around 30 miles per hour, say, will require tens of injections to be made into each of its engine cylinders per second.
Research has shown that, for a given injector, significant improvements in combustion quality and efficiency are achieved by rapidly varying the injection pressure, or injection rate, within an injection event. More particularly, commencing the event with a pilot injection at lower pressure and then following that with a main injection at higher pressure (as shown in the example injection profile of FIG. 1a), has highly beneficial effects on combustion emissions and noise. The benefits to emissions are still further improved if the main injection has a so-called ‘boot-shaped’ injection profile (an example of which is shown in FIG. 1b), whereby the main injection commences at a pilot pressure P1 but is then rapidly increased to a significantly higher pressure P2.
The timing of when fuel is supplied to an engine cylinder is determined by the movement of an injection valve needle of the associated injector. A known injector 200 is shown schematically in FIG. 2a. An electronically operated three-way needle control valve 202 is provided for controlling movement of an injector valve needle 204, relative to its seating 206, thereby to control the delivery of fuel from a fuel delivery chamber 207 to the engine cylinder (the engine cylinder is not shown in FIG. 2a). The needle control valve 202 is operable either to connect a high pressure (HP) supply line 208 and a control chamber 210 which houses the non-seating end of the valve needle 204, or to connect the control chamber 210 with a low pressure (LP) reservoir. The high pressure supply line 208 also supplies the fuel delivery chamber 207 which houses the seating end of the valve needle 204.
When the needle control valve 202 is in its ‘closed’ position (as shown in FIG. 2a), the control chamber 210 communicates with the high pressure supply line 208 such that it floods with high pressure fuel. This fuel, together with a spring 212 located in the control chamber 210, serves to urge the valve needle 204 against its seating 206 so that no fuel enters the cylinder. In contrast, when the needle control valve 202 is ‘open’ (as shown in FIG. 2b), communication between the control chamber 210 and the high pressure supply line 208 ceases and the control chamber 210 communicates instead with the low pressure reservoir. The valve needle 204 experiences so-called thrust forces arising from the high pressure fuel surrounding it (as shown in FIG. 3 prior to the needle 204 lifting). As pressure within the control chamber 210 reduces, these thrust forces become sufficient to overcome the force of the control chamber spring 212 and the valve needle 204 lifts from its seating 206, whilst the injector 200 continues to be supplied by the high pressure supply line 208. Accordingly, fuel is injected into the engine cylinder, through the revealed apertures, during an injection.
With regard to the desired boot injection profile, one way of increasing the pressure, or rate, at which fuel is injected for a given injector 200 is to use a pumping arrangement. Prior to engagement of the pumping action, a control valve is used to close communication with the high pressure fuel supply line 208. A known fuel injection system 400 of this type, devised by the present applicant and described in WO 03/093671, will now briefly be described with reference to FIG. 4.
The fuel injection system 400 is comprised of a high pressure fuel supply means 402 including a fuel pump 408 and an accumulator volume 410 which stores fuel at a moderately high and injectable pressure (for example, 300 bar). The accumulator volume 410 is more commonly referred to as a ‘common rail’. The high pressure fuel supply means 402 supplies fuel to a pumping arrangement 404 and the fuel injector 200 of the system, as described above, via a high pressure supply line 208. In between the high pressure fuel supply 402 and the pumping arrangement 404, the high pressure supply line 208 is provided with a supply control valve 406. The supply control valve 406 and fuel injector 200 are both electronically operable and are controlled by an associated electronic engine controller (not shown) which controls operation of the supply control valve 406 and the action of the pumping arrangement 404 during injections.
The pumping arrangement 404 and the fuel injector 200 are arranged within a common unit, forming a so-called electronic unit injector (EUI). An engine is typically provided with a plurality of EUIs, one for each cylinder. The pumping arrangement 404 is comprised of a pumping chamber 414 and an associated spring-loaded plunger 416 which is operated by a driven cam 418, such that the plunger 416 is driven back and forth within the pumping chamber 414 whilst the engine is operating. The driven cam 418 has a lobed profile, comprising rising and trailing flanks. When engaged with the rising flank, the plunger 416 is driven into the pumping chamber 414 in a pumping stroke and then recedes, when engaged with the falling flank, during a return stroke. In between strokes, during the remainder of the cam cycle, the plunger 416 is engaged by the substantially cylindrical body portion of the cam and remains substantially retracted with respect to the pumping chamber 414. When not injecting, the rail control valve 406 is kept in its ‘open’ position and the movement of the plunger 416 has substantially no effect on the pressure of fuel delivered via the high pressure supply line 208.
Details of how the fuel injection system 400 can be used to generate injection events having boot-shaped profiles will now be described. The electronic engine controller commences an injection event by moving the needle control valve 202 into its open position whilst also keeping the rail control valve 406 open. The needle control valve 202 is quickly returned to its closed position, such that the pilot part of the injection profile is achieved. After a short pause, the electronic engine controller reopens the needle control valve 202 to commence the main injection. Once the ‘toe’ part of the main injection profile is established at rail pressure, the electronic engine controller closes the rail control valve 406 to effect the second part of the boot profile, this action coinciding with a pump stroke by the plunger 416 when it is engaged with the rising flank of the driven cam 418.
Whilst the rail control valve 406 remains closed, the operation of the pumping arrangement 404 serves to increase the pressure of the fuel contained within the electronic unit injector by decreasing the volume in which the fuel is contained, giving rise to a peak boot pressure. The electronic engine controller ends the main injection, and hence the injection event, at or before the conclusion of the pumping stroke by returning the needle control valve 202 to its closed position and reopening the rail control valve 406. Alternatively, the main injection may be terminated by reopening the rail control valve 406, followed by the needle control valve 202 being returned to its closed position.
FIG. 5 shows a variety of main injection profile results 500 obtained using the fuel injection system 400 for an injection event of fixed duration. Of these profiles, the maximum injection pressure is shown by the ‘square’ profile 502. Prior to injecting, the rail control valve 406 has been closed for a substantial part of a pumping stroke of the plunger 416, such that when the needle control valve 202 is moved into its open position the fuel within the electronic unit injector has undergone the maximum compression available within the fuel injection system 400. In contrast, the profile 504 closest in character to the desired boot profile, which is obtained as explained above, achieves the lowest peak pressure of the profiles shown. This is because the injection event initially proceeds with the rail control valve 406 open, leaving less time (in an injection event of fixed quantity) for a build-up of pressure to be achieved under the action of the plunger 416.
In order to optimise emissions from an engine operating under a particular load condition, it is desirable to have a fuel injection system that can readily implement variations to the injection profile. Accordingly, one can then vary the constituent parts of the profile and measure the effects on the emissions. However, it is thought that the benefits on emissions arising from a boot injection profile are only realised when the peak pressure is sufficiently high. In addition, any injection profile is subject to certain practical constraints, namely a minimum volume of fuel must be delivered within a particular time frame that coincides with the cam cycle. Whilst the fuel injection system 400 has great advantages for allowing boot shaped injections to be realised, in some circumstances the system does not facilitate variations on an optimum scale.
It is therefore an object of the present invention to provide an improved injection system to address the issues mentioned above. More specifically, it is desired to provide a fuel injection system which allows greater control over the profile characteristics of an injection event, whilst allowing a sufficient quantity of fuel to be injected.